Information recording medium and method for manufacturing the same

ABSTRACT

An information recording medium having such a recording material layer on a substrate where reversible phase change between electrically or optically detectable states can be caused by electric energy or electromagnetic energy. The recording material forming the recording layer is either a material having a crystal structure including lattice defects in one phase of the reversible phase change or a material having a complex phase composed of a crystal portion including a lattice defect in one phase of the reversible phase change and an amorphous portion. Both portions contain a common element. A part of the lattice defects are filled with an element other than the element constituting the crystal structure. The recording medium having a recording thin film exhibits little variation of the recording and reproduction characteristics even after repetition of recording and reproduction, excellent weatherability, strong resistance against composition variation, and easily controllable characteristics.

TECHNICAL FIELD

The present invention relates to an information recording medium thatcan record, reproduce, erase and rewrite high-density information bymeans of irradiation of laser beams and application of a high electricfield. The present invention relates to also a method for manufacturingthe information recording medium.

BACKGROUND ART

It is well known to apply as a memory a change in opticalcharacteristics caused by reversible phase change of a substance, and atechnique using this has come into practice as phase change opticaldisks such as DVD-RAM. Specifically, recording, reproducing andrewriting of signals will be available by rotating a disk mediumcomprising a substrate on which a recording thin film for generatingreversible phase change is provided, and by irradiating the disk mediumwith a laser beam drawn to a sub-micron size. In the case of a phasechange optical disk, overwriting by means of a single laser beam iscarried out. That is, irradiation is performed by modulating the laserpower between a high level and a low level depending on the informationsignal, so that an amorphous phase is generated at a region irradiatedwith a high power laser beam while a crystalline phase is generated at aregion irradiated with a low power laser beam. As a result, a signalarray comprising the amorphous portion and crystal portion alternatelyis recorded on the disk. Since the amorphous portion and the crystalportion are different in the light transmittance and reflectance, thechange in the state can be read as a change in the amount of the lighttransmittance or reflectance by continuously irradiating a laser beam onthis signal array, in which the laser beam is attenuated not to changethe recording film.

Such a phase change optical disk has some characteristics such as:

-   -   (1) it enables the performance of overwriting, i.e., recording a        new signal while erasing an old signal by using only one laser        beam; and    -   (2) it can record and reproduce a signal by using a change in        the reflectance, based on a principle similar to that of a ROM        medium. These characteristics lead to several merits such as        simplifying a system construction and providing devices for        general purposes, so that such phase change optical disks are        expected to be applied widely.

Recording materials used for recording layers of phase change opticaldisks generally include chalcogenide semiconductor thin films based onchalcogen elements such as Te, Se and S. A method used in the early1970s is crosslinking a Te network structure for stabilizing anamorphous state by adding materials such as Ge, Si, As and Sb to a maincomponent of Te. However, these materials would cause a problem. Thatis, when the crystallization temperature is raised, the crystallizationspeed is lowered remarkably, and this would make rewriting difficult.Alternatively, when the crystallization speed is increased, thecrystallization temperature is lowered sharply, and thus, the amorphousstate will be unstable at a room temperature. A technique suggested forsolving the problems in the latter half of the 1980s is the applicationof a stoichiometric compound composition. The thus developedcompositions include Ge—Sb—Te based materials, In—Sb—Te based materials,and GeTe based materials. Among them, Ge—Sb—Te based materials have beenstudied most since the materials allow phase change at high speed,substantially no holes will be formed even after repeated phase changes,and substantially no phase separation or segregation will occur (N.Yamada et al, Jpn. J. Appl. Phys.26, Suppl. 26-4, 61 (1987)). An exampleof material compositions other than such stoichiometric compositions isan Ag—In—Sb—Te based material. Though this material is reported to beexcellent in the erasing performance, it has been found that thecharacteristics deteriorate due to the phase separation as a result ofrepeated overwriting.

Similarly, characteristic deterioration caused by repetition may beobserved even if a stoichiometric composition is used. An example of thedeterioration mechanism is a phenomenon of micro-scaled mass transfercaused by repetition of overwriting. More specifically, overwritingcauses a phenomenon that substances composing a recording film flowlittle by little in a certain direction. As a result, the film thicknesswill be uneven at some parts after a big repetition. Techniques tosuppress the phenomenon include the addition of additives to recordinglayers. An example of such techniques is addition of a N₂ gas at a timeof film formation (JP-A-4-10979). A document clarifies a mechanism thata nitride having a high melting point is deposited like a network in agrain boundary composing the recording film, and this suppresses theflow (R. Kojima et al. Jpn. J. Appl. Phys. 37 Pt.1, No. 4B. 2098(1998)).

JP-A-8-127176 suggests a method of including a material having a meltingpoint higher than that of the recording material.

As mentioned later, the cited reference is distinguishable from thepresent invention in that the material having a high melting point willnot be dissolved in the base material but scattered in the base materiallayer. According to the reference, the scattered material having a highmelting point suppresses the mass transfer phenomenon caused by repeatedoverwriting so as to improve the performance. JP-A-7-214913 suggests,without clarifying the mechanism, the addition of small amounts of Pt,Au, Cu, and Ni in a Ge—Sb—Te film in order to improve stability of theamorphous phase without lowering the repeatability.

However, the repetition number tends to decrease when the recordingdensity is increased. Due to a recent demand for keeping compatibilityamong media of various generations, recording at higher density shouldbe performed by using optical heads of identical performance (i.e.,laser beams of an identical wavelength and object lenses of an identicalnumerical aperture). The size of a recording mark should be reduced toraise recording density. On the other hand, the strength of reproducedsignals is lowered as the recording mark becomes small, and the signalswill be influenced easily by a noise. Namely, during a repeatedrecording, even a slight variation that may have not caused a trouble ina conventional process will lead to errors in reading, and thus, thenumber of available repetitions of rewriting is decreased substantially.This problem can be noticeable in the a case of so-called land-grooverecording, in which a concave-convex-shaped groove track is formed on asubstrate and information is recorded on both the groove (a regioncloser to the light-incident side) and the land portion (spacing betweenthe grooves) in order to guide a laser beam for recording andreproducing. Specifically, since the thermal and optical conditions aredifferent between the land and groove, the repeatability willdeteriorate easily, especially in the land region.

Merits provided by a recording layer comprising a compound material havebeen described above. On the other hand, when the composition of therecording layer is changed from the stoichiometric composition, therecording performance will be changed remarkably. In a desirablerecording method, the performance of a recording film should becontrolled with further accuracy while keeping the merits of thecompound composition, and using an identical recording film or acomposition having a wide acceptability with respect to characteristics.

Electrical switching devices comprising a chalcogenide material andmemory devices are known as well as applications of such phase changematerials. The electrical phenomenon was first reported in 1968.Specifically, when voltage is applied gradually to a phase changematerial thin film in an as-depo.-state sandwiched between electrodes,electrical resistance between the electrodes sharply declines at acertain threshold voltage, and a large current will start to flow(crystallization). For reversing this state to an initial low-resistantstate (OFF state), a big and short current pulse will be passed. Aportion provided with current is melted first and then, quenched to beamorphous so that the electrical resistance is increased. Sincedifferences in the electrical resistance can be detected easily by anordinary electrical means, the material can be used as a rewritablememory. Though material compositions based on Te have been used forelectrical memories, any of them require a μs order period of time forcrystallization.

DISCLOSURE OF INVENTION

To solve the above-mentioned problems, a first purpose of the presentinvention is to provide a phase change memory material that willincrease a number of repetitions of rewriting and enables rewriting at ahigh speed. The memory device can be constituted with either an opticalmemory or an electric memory. The present invention aims to provide arecording medium comprising a recording thin film formed on a substrate.Due to the above-mentioned excellent characteristics of stoichiometriccomposition, the recording thin film provides less influence on thecharacteristics regardless of some composition variation. That is, therecording thin film comprises a composition exhibiting easycontrollability of the characteristics. The present invention providesalso a method for manufacturing a recording medium comprising such arecording thin film.

For achieving the purposes, an information recording medium according tothe present invention comprises a recording material layer formed on asubstrate, and the recording material layer enables the generation ofreversible phase change by means of electric energy or electromagneticwave energy in an electrically or optically detectable state. Theinformation recording medium is characterized in that the recordingmaterial layer is composed of either a material having a crystalstructure including lattice defects in one phase of the reversible phasechange (material ‘A’) or a material in a complex phase comprisinglattice defects in one phase of the reversible phase change comprising acrystal portion and an amorphous portion, and both the portions comprisea common element (material ‘B’), and that at least one part of theabove-mentioned lattice defects is filled with an element other than theelements composing the above-mentioned crystal structure.

Next, a method for manufacturing an information recording mediumaccording to the present invention relates to an information recordingmedium comprising a recording material layer formed on a substrate, andthe recording material layer generates reversible phase change by meansof electric energy or electromagnetic wave energy in an electrically oroptically detectable state. It is characterized in that the recordinglayer is constituted with a recording material having a crystalstructure in which one phase of the reversible phase change includeslattice defects, and that at least a part of the defects is filled withadditional elements.

The present invention employs the following material compositions forgenerating reversible phase change between an amorphous phase and acrystalline phase by irradiating the material layer with a laser beam orenergizing the same layer. The material composition forms a single phaseduring crystallization and the crystal lattice necessarily includes somedefects. At least a part of the lattice defects is filled with anelement other than the element composing the base material in order toexhibit a new compound phase that has never been observed. Fillingadditional elements in the lattice of the base material can change thecharacteristics of the base material fundamentally.

For solving the above-mentioned problems, the present invention employsan amorphous material layer to be crystallized by irradiating a laserbeam or by energizing. The material phase forms a complex phase(crystalline phase) comprising a compound phase portion having latticedefects within the crystal and an amorphous phase portion. Here, it isimportant and preferred that the compound phase portion is filled withadditional elements, and the amorphous phase is a single phase. It ispreferable that a molar ratio of the amorphous phase to the crystallinephase in the complex phase is 2.0 at most, and further preferably, theratio is 1.0 at most.

Regardless whether the crystalline phase is a single phase or a complexphase, it is preferable that the compound comprises a base material ofrock-salt type structure (NaCl) having a crystal structure with alattice defect (vacancy). As mentioned above, at least one part of thelattice defects included in the base material is filled with an atomother than elements composing basic substances of the rock-salt typestructure. It is preferable for the element to fill the lattice defectsthat Rim is closer to Rnc, e.g., 0.7<Rim≦1.05 Rnc, where Rim denotes anionic radius of an element to fill the lattice defects, and Rnc denotesan ionic radius of a smallest ion among elements composing the rock-salttype crystal. When Tim denotes a melting point of an element to fill thelattice defects and Tnc denotes a melting point of the rock-salt typecrystal, it is preferable that the Tim is closer to Tnc, i.e., therelationship satisfies |Tim−Tnc|≦100° C. When Dim denotes aconcentration of an element added to fill the lattice defects and Ddfdenotes a concentration of the lattice defects in the rock-salt typecrystal, it is preferable that Dim≦Ddf×1.5. It is further preferablethat 0.2≦Dim≦Ddf.

Specifically, the material is preferred to contain Te. A substance toform the amorphous phase in the complex phase comprises at least one ofSb, Bi, In, Ge and Si. At least a part of the elements can comprise anoxide, a nitride, a fluoride, and a nitride-oxide. It should be notedhere that the compound phase and the amorphous phase preferably containa common element. For example, when an element composing the crystallinephase is based on three elements of Ge, Sb and Te, the amorphous phaseis preferred to contain Sb or Ge as a main component. Alternatively, itis further preferable that the compound phase contains Ge, Sb and/or Biand Te while the amorphous phase contains Sb and/or Bi or Ge. It ispreferable that at least one element selected from Sn, Cr, Mn, Pb, Ag,Al, In, Se and Mo is included in the crystalline phase.

The element composing the rock-salt type crystal preferably contains Geand Te as its base materials, and further preferably, it contains atleast one element selected from Sb and Bi. It is particularly preferablethat the base material composition of the rock-salt type crystalsubstantially corresponds to a GeTe—Sb₂Te₃ quasibinary systemcomposition, a GeTe—Bi₂Te₃ quasibinary system composition or a mixturethereof. When an element composing the rock-salt type crystal containsGe, Te, and Sb, or it contains Ge, Te, and Bi, the element to fill thelattice defects is at least one selected from Al, Ag, Pb, Sn, Cr, Mn andMo. It is also preferable that the base material composition of therock-salt type crystal substantially corresponds with(GeTe)_(1−x)(M₂Te₃)_(x), in which 0.2≦x≦0.9 (M denotes at least oneelement selected from Sb, Bi and Al, or an arbitrary mixture of theseelements). It is further preferable that 0.5≦x≦0.9. For improvingrecording sensitivity, it is further preferable that the recording filmcontains nitrogen (N) or oxygen (O). Preferably, the concentration ofthe N atom (Dn) is 0.5 atom %≦Dn≦5 atom % since the range provideshigher effects.

Filling Al, Cr or Mn in lattices is preferable to improve repeatability,and addition of Ag is preferable to increase changes in opticalcharacteristics (signal amplitude change) between the crystalline phaseand the amorphous phase. Filling Sn or Pb is effective in improvingcrystallization speed.

It is further effective to fill plural elements at the same time inlattice defects for improving the characteristics. When the material isbased on Ge—Sb—Te or Ge—Bi—Te, both the crystallization speed and therepeatability can be improved preferably at the same time by, forexample, using simultaneously at least one of Sn and Pb together withAl, Cr or Mn. Otherwise, simultaneous use of either Sn or Pb togetherwith Ag is preferable to improve the crystallization speed and thesignal amplitude at the same time. Using at least one of Al, Cr and Mntogether with Ag is preferable to improve repeatability and signalamplitude at the same time. Furthermore, addition of at least one of Al,Cr and Mn, at least either Sn or Pn together with Ag is preferable inimproving crystallization speed, signal amplitude and repeatability atthe same time.

Preferably, such a material layer is manufactured by lamination such asvapor deposition and sputtering. Specifically, it is further preferablethat sputtering is carried out by using a target including a componentcomposing the rock-salt type crystal and an element to fill the latticedefects. Preferably, the target contains at least Ge and Te as elementsfor forming the rock-salt type crystal, and further preferably, containsan element selected from Al, Sb and Bi. Especially preferable elementsto fill the lattice defects include Ag, Sn, Pb, Al, Cr, In, Mn and Mo.It is further preferable that sputtering is carried out in a gaseousatmosphere containing Ar and N₂. It is also preferable that thesputtering gas contains at least one gas selected from N₂ gas and O₂gas.

An optical information recording medium according to the presentinvention can comprise a single layer medium prepared by forming theabove-mentioned recording material thin film on a substrate. However, itis desirable to use a multilayer including the recording layer. Forexample, it is preferable that a protective layer is provided betweenthe substrate and the recording layer in order to reduce thermal damagein the substrate or to utilize its optical interference effect. It isalso preferable to provide a protective layer to the opposing surface ofthe recording layer as well in order to prevent deformation of therecording layer and to utilize its optical interference effect. Theprotective layer is made of a material that is stable thermally andchemically, and transparent optically, such as an oxide, a sulfide, anitride, a nitride-oxide, a carbide, and fluoride. Examples of thematerials include ZnS, SiO₂, ZnS—SiO₂, SiNO, SiN, SiC, GeN, Cr₂O₃, andAl₂O₃. It is preferable to provide a reflecting layer over theprotective layer in order to increase efficiency for laser beams or thelike used for recording. The reflecting layer can be a metallic materialfilm or a multilayer film combined with a dielectric material. Themetallic material can be Au, Al, Ag or an alloy based on these metals.

An electric information recording medium according to the presentinvention can be constituted by laminating sequentially on a substratean electrode material, the above-mentioned material thin film, and afurther electrode material. Otherwise, such a medium can be constitutedby laminating the material thin film and an electrode material on ametallic substrate that functions also as an electrode.

Materials of the respective layers are formed by lamination such assputtering and vapor deposition similar to the case of an opticalinformation recording medium. Since an electric memory system in thepresent invention causes variation in electrical resistance, it can beused as a component for a variable programmable circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view to show a structure (atom position at a timeof crystallization) of a representative recording film used for aninformation recording medium of the present invention, in which thecrystalline phase is a single phase. In this example, the crystallinephase is constituted with a single compound phase (moreover, it is arock-salt type structure). In the lattice site position forming therock-salt type structure, all 4 a sites are occupied by Te atoms 1,while 4 b sites are occupied by Ge atoms 2, Sb atoms 3, and occupied byalso lattice defects 4. In the present invention, atoms other than theatoms occupying the 4 b sites are filled in the lattice defects.

FIG. 2 is a schematic view to show a structure (atom position at a timeof crystallization) of another representative recording film used for aninformation recording medium of the present invention, in which therecording layer is a complex phase (a crystalline phase). In FIG. 2, (a)denotes a crystalline phase 100. The crystalline phase is a complexphase (mixture phase) 100 comprising a component 110 having a compoundstructure basically equal to that shown in FIG. 1 and also an amorphouscomponent 120. In FIG. 2, (b) denotes an amorphous phase 200. In (b), asingle phase is formed.

FIGS. 3A-3D are further specific examples of the structure shown in FIG.2.

FIGS. 4A-4J are cross-sectional views of an example of a layerconstitution of an optical information recording medium according to thepresent invention. In FIGS. 4A-4J, 7 denotes a substrate, 8 denotes arecording layer (phase change material layer), and 9 and 10 denoteprotective layers. Numeral 11 denotes a reflective layer, 12 denotes anovercoat layer, 13 denotes an adhesive layer, and 14 denotes aprotective plate. Numeral 15 denotes a surface layer, 16 and 17 denoteinterface layers, 18 denotes an optical absorption layer, 19 denotes areflective layer (light incident side), and 20 and 21 respectivelydenote multilayer films of the above-mentioned thin films.

FIG. 5 is a schematic view of a crystal structure to show positions ofadditional elements in the crystalline phase of a recording film usedfor an information recording medium according to the present invention.Numeral 22 denotes a position of an atom filling a lattice defect in arock-salt type crystal lattice.

FIGS. 6A-6C are graphs to show laser modulation waveforms to evaluatethe recording performance of an optical information recording mediumaccording to the present invention. FIG. 6A shows the recordingperformance regarding a 3T pulse, FIG. 6B shows the recordingperformance regarding a 4T pulse, and FIG. 6C shows the recordingperformance regarding 5T-11T pulses.

FIG. 7 is a graph to show a relationship between a proper additiveconcentration and a lattice defect concentration in an informationrecording medium according to the present invention.

FIGS. 8A-8F and 9A-9E show examples of crystal structures of recordingfilms used for information recording media according to the presentinvention. The respective structures will cope with any compound phasesshown in FIGS. 1 and 2.

FIG. 10 is a schematic view to show a basic structure of an electricmemory device (a reversible change memory of a resistor) according tothe present invention. In FIG. 10, 23 denotes a substrate, 24 and 27denote electrodes, 25 denotes an insulator, 26 denotes a phase changematerial film, 28 and 29 denote switches, 30 denotes a pulse powersource, and 31 denotes an electrical resistance meter.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 2 is a cross sectional view to show an example (layer constitution)of an optical information recording medium according to the presentinvention. A typical information recording medium is constituted byforming a recording layer 8 having the above-mentioned constitution on asubstrate 7 selected from transparent polycarbonate resin, an acrylicresin, a polyolefin-based resin, a glass sheet or the like. Protectivelayers 9 and 10 can be formed on at least one surface of the recordinglayer. Reflective layers 11 can be formed on the respective protectivelayers. Overcoats 12 can be formed on the top layers, or the overcoatscan be replaced by protective plates 14 that are adhered by adhesivelayers 13. For guiding laser beams used in recording/reproducing, aspiral or concentric circular concave-convex groove track, a pit array,a track address can be formed on the substrate surface. Such a recordingmedium is irradiated with a laser beam in order to cause reversiblephase change in the recording layer between a crystalline phase and anamorphous phase, so that information can be rewritten. In the case ofcrystallization, the recording medium is irradiated with a laser beamlike a pulse in order to keep the irradiated part at or above an interimcrystallization change temperature. In changing the recording layer tobe amorphous, the layer is irradiated with a more intensive laser beamfor a period equal to or shorter when compared to a case ofcrystallization, so that the irradiated part is melted instantaneouslyand then quenched. This reversible phase change can be detected as achange in the reflectance or transmittance. This reproduction is carriedout by irradiating the recording medium with a laser beam weakened notto provide any additional influence so as to detect changes in thestrength of light reflected from the irradiated portion or transmitted.

An optical information recording medium according to the presentinvention, as shown in FIGS. 4A-4J, will be characterized by acomposition of a material composing the recording layer 8 and by theinternal structure. A representative example will be explained belowwith reference to a Ge—Sb—Te based material. As reported in N. Yamada etal., J. Appl. Phy.69(5), 2849 (1991),a Ge—Sb—Te material is crystallizedto have a face-centered cubic structure meta-stably by irradiating alaser beam. In addition to that, a recent research presentation by thesame author (MRS-Buttetin, 21(9), 48(1996) and a research presentationby Nonaka et al. (papers for the tenth symposium on phase changerecording, p.63) suggest that the metastable phase necessarily containsmany lattice defects (vacancy). The following description is about arepresentative composition of a stoichiometric compound composition ofGe₂Sb₂Te₅. The material has a metastable phase of rock-salt type (NaCltype). As shown in FIG. 1, all lattice positions (4 a sites)corresponding to Cl atoms are occupied by Te atoms 1, and all latticesite positions (4 b sites) corresponding to Na atoms are occupied by Geatoms 2 and Sb atoms 3 at random depending on the composition ratio.However, since the total number of the Ge atoms and the Sb atoms isgreater than the number of the Te atoms, the 4 a sites necessarily haslattice defects 4 of about 20% (about 10% of the entire sites). Thelattice defects also are located at random (An example of atom positionsin 4 a sites is shown).

The inventors reported that such a Ge—Sb—Te system makes a crystalhaving a substantially identical face-centered-cubic crystal structureeven if the composition is changed. Recent studies show that a Sb atomdoes not enter a crystal lattice but an added Sb atom exists in aseparate structure on an interface of a crystal particle even if Sb isincluded in a form of, e.g., Ge₂Sb_(2+x)Te₅ (0<x≦1) to fill the defects.Particularly, the Sb atom will exist in an amorphous phase especiallyfor a case of laser crystallization. Specifically, the result ofobservation by a detailed X-ray diffraction demonstrates that even if Sbis added to a stoichiometric composition Ge₂Sb₂Te₅ thin film, the Sbatom does not enter the crystal lattice to fill the lattice defectcompletely. As a result, Ge₂Sb₂Te₅ crystal and Sb will coexist in astructure of a recording film in a crystalline state. In a typical caseof two-phase coexistent composition, repetition of amelting-solidification process will cause a phase separation, and thiswill lead to local variation in the composition. An advantage of thiscase is that such a phase separation will not proceed since the meltingpoint of Sb is considerably close to that of Ge—Sb—Te and since theGe—Sb—Te also includes Sb.

Besides Sb, some additives can prevent crystal growth though theconditions vary in many cases. For example, JP-A-7-214913 discloses theaddition of Pd. This reference discloses that crystallization becomesdifficult when the amount of the additives exceeds 2 atom %. From thefact that a very small amount of additive causes an abrupt change in thecharacteristics, Pd is considered to exist without entering the latticedefects. In other words, even a small amount of Pd is considered to beseparated completely from Ge—Sb—Te but not to enter a crystal latticebased on Ge—Sb—Te. However, when the Pd concentration reaches about 2atom %, characteristics of Pd as a material having a high-melting pointbecome remarkable, and the Pd will restrict the movement of atoms so asto substantially prevent crystallization. Moreover, repetition ofrecording and erasing accelerates phase separation of the Ge—Sb—Te andPd. In other words, an additive that does not enter a lattice cannot besuitable for controlling the characteristics.

On the other hand, a relatively easy relationship between Sbconcentration and change in the crystallization characteristicsfacilitates control of the characteristics and serves to maintain highrepeatability. This fact may suggest that the melting point of anadditional element cannot be too much higher than that of the basematerial in order to change the characteristics widely and continuouslyby adding the element. It is also desirable that the additional elementcan enter the crystal lattice and especially, the element does notcreate a separate crystalline phase. A further merit is that entering ofexcessive and harmful atoms can be prevented by previously filling thelattice defects with useful atoms.

The inventors evaluated recording materials from the above-mentionedaspects and found that additional elements enter crystal lattices andthus characteristics can be controlled continuously with high accuracyunder a certain condition. The inventors found also that some additiveswill take place of elements of the base material. Moreover, theadditives may change the purged elements. In addition, the temperatureand speed of crystallization can be controlled by controlling thecondition and concentration of the purged elements, and this will leadto desirable recording/erasing performance. It is reasonable that inthis case, a part of elements forming a compound in a crystal is commonto elements that have been purged outside the compound and exist in anamorphous phase in the grain boundary or the like. This means thatpositional uniformity of the composition will be maintained easily allthe time that phase changes between a crystalline phase and an amorphousphase occur. Specifically, the additives prevent the progress of phaseseparation even when the crystalline phase becomes a complex phase, andthus, good repeatability can be maintained. It can be concluded from theabove facts that a material being a single phase and necessarilyincluding lattice defects can provide unexpected characteristics byfilling the lattice defects appropriately with any other atoms. Also, itis suggested that addition of a certain element can help formation of amaterial having a new structure.

The following explanation is about a specific material composition toconstitute a recording layer 8. A primary condition for a material inthe present invention is to obtain a material comprising many latticedefects. A crystalline phase comprising lattice defects will appear as ametastable phase in materials that can be represented by GeTe-M₂Te₃ (Mis, for example, Sb, Bi or Al). The examples are a Ge—Sb—Te basedmaterial comprising a GeTe—Sb₂Te₃ composition, a Ge—Bi—Te materialcomprising a GeTe—Bi₂Te₃ based composition, or a Ge—Te—Al based materialcomprising a GeTe—Al₂Te₃ based composition. Similarly, a crystallinephase including lattice defects will appear as a metastable phase incompositions of the mixtures such as Ge—Sb—Bi—Te, Ge—Sb—Al—Te,Ge—Bi—Al—Te, and Ge—Sb—Bi—Al—Te. Similar constitutions are obtained forGe(Te,Se)-M₂(Te,Se)₃ in which a part of Te is replaced by Se. Theexamples are Ge—Te—Se—Sb, Ge—Te—Se—Bi, Ge—Te—Se—Sb—Bi, Ge—Te—Se—Al,Ge—Te—Se—Sb—Al, Ge—Te—Se—Bi—Al, and Ge—Te—Se—Sb—Bi—Al. Similar effectswere obtained by applying, for example, Ge—Sn—Te—Sb, Ge—Sn—Te—Sb—Al,Ge—Pb—Te—Sb, and Ge—Pb—Te—Sb—Al, which are obtained by substituting apart of the Ge with Sn or with Pb. Similar constitutions were obtainedwhen N was added to the compositions. These are crystallized meta-stablyto have a face-centered-cubic crystal structure (rock-salt structure).When the 4 b sites of the rock-salt type structure are occupied by Te(or Se) and the 4 a sites are occupied by other element M as mentionedabove, Te (or Se) atoms outnumber M atoms, which will create latticedefects at the 4 a sites inevitably. The lattice defects cannot befilled completely with the above-mentioned elements such as Sb. Thereason has not been clarified yet, but it can be deduced that ametastable phase of a rock-salt type cannot be formed without a certainnumber of lattice defects inside thereof. Namely, filling the defectsmay raise the entire energy so that the rock-salt type structure cannotbe kept.

As a result of various analyses and experiments, the inventors havefound that not all elements can fill lattice defects and that an ionicradius is an important factor to determine the conditions. When the 4 asites have lattice defects, the defected lattices of the base materialswill be filled easily if Rim is sufficiently close to Rnc, where Rncdenotes an ionic radius of an element having a minimum ionic radiusamong elements occupying the 4 a sites and Rim denotes an ionic radiusof an additional element. According to Third Revision of Manual of BasicChemistry (Kagaku-binran Kiso-hen) II issued by Maruzen Co., Ltd., theradius of a Ge⁴⁺ ion is 0.67, the radius of a Sb⁵⁺ ion is 0.74 μm, andthe radius of a Te²⁻ ion is 2.07 μm when the coordination number is 6.For Ge—Sb—Te, an element can enter a lattice easily when it has an ionicradius substantially the same or slightly smaller than the radius of aGe ion located at a 4 a site. Each Ge ion has a smaller ionic radiusthan that of a Sb ion. TABLE 1 Ionic radii and element's melting pointsfor respective ion species Ion species with a Element's coordinationIonic radius melting point No. number of 6 (nm) (° C.) 1 N⁵⁺ 2.7 −209.862 V⁵⁺ 5.0 1890 3 S⁴⁺ 5.1 112.8 4 Si⁴⁺ 5.4 1410 5 P³⁺ 5.8 44.1 6 Be²⁺ 5.91280 7 As⁵⁺ 6.0 817 8 Se⁴⁺ 6.4 217 9 Ge⁴⁺ 6.7 937.4 10 Mn⁴⁺ 6.7 1240 11Re⁷⁺ 6.7 3180 12 Al³⁺ 6.8 660.37 13 Co^(3+l) 6.9 1490 14 Fe^(3+l) 6.91540 15 Cr⁴⁺ 6.9 1860 16 Re⁶⁺ 6.9 3180 17 Te⁶⁺ 7.0 449.5 18 Ni^(3+l) 7.01450 19 As³⁺ 7.2 817 20 Mn^(3+l) 7.2 1240 21 V⁴⁺ 7.2 1890 22 Mo⁶⁺ 7.32620 23 Sb⁵⁺ 7.4 630.74 24 Ni^(3+h) 7.4 1450 25 Rh⁴⁺ 7.4 1970 26 W⁶⁺ 7.43400 27 Co^(3+h) 7.5 1490 28 Fe^(2+l) 7.5 1540 29 Ti⁴⁺ 7.5 1660 30 Mo⁵⁺7.5 2620 31 Ga³⁺ 7.6 29.78 32 Pd⁴⁺ 7.6 1550 33 Cr³⁺ 7.6 1860 34 Ru⁴⁺ 7.62310 35 W⁵⁺ 7.6 3400 36 Pt⁴⁺ 7.7 1770 37 Ir⁴⁺ 7.7 2410 38 0s⁴⁺ 7.7 304539 V³⁺ 7.8 1890 40 Nb⁵⁺ 7.8 2470 41 Ta⁵⁺ 7.8 2990 42 Mn^(3+h) 7.9 124043 Co^(2+l) 7.9 1490 44 Fe^(3+h) 7.9 1540 45 Tc⁴⁺ 7.9 2170 46 Mo⁴⁺ 7.92620 47 W⁴⁺ 8.0 3400 48 Mn^(2+l) 8.1 1240 49 Ti³⁺ 8.1 1660 50 Rh³⁺ 8.11970 51 Ru³⁺ 8.2 2310 52 Ir³⁺ 8.2 2410 53 Nb⁴⁺ 8.2 2470 54 Ta⁴⁺ 8.2 299055 Sn⁴⁺ 8.3 231.96 56 Ni²⁺ 8.3 1450 57 Mo³⁺ 8.3 2620 58 Hf⁴⁺ 8.5 2230 59Mg²⁺ 8.6 648.8 60 Zr⁴⁺ 8.6 1850 61 Nb³⁺ 8.6 2470 62 Ta³⁺ 8.6 2990 63Ge²⁺ 8.7 937.4 64 Cu²⁺ 8.7 1083.4 65 U⁵⁺ 8.7 1132.3 66 Cr^(2+l) 8.7 186067 Zn²⁺ 8.8 419.58 68 Sc³⁺ 8.8 1540 69 Co^(2+h) 8.9 1490 70 Li⁺ 9.0180.54 71 Bi⁶⁺ 9.0 271.3 72 Sb³⁺ 9.0 630.74 73 Pd³⁺ 9.0 1550 74 Cu⁺ 9.11083.4 75 Pb⁴⁺ 9.2 327.502 76 Fe^(2+h) 9.2 1540 77 V²⁺ 9.3 1890 78 In³⁺9.4 156.61 79 pt²⁺ 9.4 1770 80 Cr^(2+h) 9.4 1860

Atoms in a rock-salt structure are considered to have a coordinationnumber of 6. Table 1 is a list of ion species each having a coordinationnumber of 6 and ionic radius of about 0.67 in an order of the ionicradius. Since a Ge⁴⁺ ion has ionic radius of 0.67, ions ranging from avanadium ion V⁵⁺ that is about 70% of a Ge⁴⁺ ion to a Ni³⁺ ion that isabout 105% may enter a lattice. That is, effective elements are V, S,Si, P, Be, As, Se, Ge, Mn, Re, Al, Co, Te, Cr, and Ni. Among them, V, S,Si, Mn, Al, Co, Cr, and Ni etc. are suitable. The remaining elements arenot suitable, since, for example, Be, As and P may cause problems due tothe toxicity, while Ge and Te compose the base material, and Re is aradioactive element.

Elements for filling lattices are not limited to the above-mentionedones. The above-mentioned condition is just one factor to determine easyaccess to a lattice. An element that composes a compound of a rock-salttype structure is observed to enter a lattice easily. Specifically, Ag,Sn and Pb were observed entering lattices, since Ag, Sn and Pb composeAgSbTe₂, SnTe, and PbTe respectively.

In addition to the suitability to fill a lattice, another importantfactor for additional elements is the melting point. Formation of anamorphous mark with a phase change optical disk requires a process ofmelting a recording film before quenching. For such a case, a meltingpoint of the additive is preferred to be close to the melting point ofan entire recording film (more preferably, a melting point of theadditive is close to melting points of all elements composing therecording film). If the additive has a melting point much higher thanthe entire melting point, phase separation will proceed easily duringrepetition of melting and solidification. In such a case, it isdifficult to keep the additives stably in lattices even when the ionicradii are closer to each other. In other words, phase separation occurs,and the phase separation creates a region comprising more additives anda region comprising fewer additives. It is preferable to decrease thedifference between the melting points, however, when the difference isabout 100° C., lattice defects can be filled while creatingsubstantially no phase separation. Otherwise an extremely uniform mixedphase can be formed even without forming a single phase. For a case ofGe₂Sb₂Te₅, the melting point is about 630° C. Therefore, an additive ispreferred to have a melting point in a range from about 530° C. to 730°C. Table 2 is a list of elements to form ions having coordination numberof 6 as mentioned above, and the elements are described sequentiallyfrom the one with a lower melting point. This table shows that elementsranging from No. 25 (Sb) to No. 31 (Ba) are within the range. That is,corresponding elements are Sb, Pu, Mg, Al and Ba, from which Pu as aradioactive element and Sb as a base material are excluded. Theremaining Mg, Al, Ba or the like are used suitably for the purpose.TABLE 2 Melting points of respective elements and ionic radii of ionspecies Ion species with a Element's coordination Ionic radius meltingpoint No. number of 6 (nm) (° C.) 1 Cs⁺ 18.1 28.4 2 Ga³⁺ 7.6 29.78 3 Rb⁺16.6 38.89 4 P³⁺ 5.8 44.1 5 K⁺ 15.2 63.65 6 Na⁺ 11.6 97.81 7 S²⁻ 17.0112.8 8 S⁴⁺ 5.1 112.8 9 I⁻ 20.6 113.5 10 In³⁺ 9.4 156.61 11 Li⁺ 9.0180.54 12 Se²⁻ 18.4 217 13 Se⁴⁺ 6.4 217 14 Sn⁴⁺ 8.3 231.96 15 Bi³⁺ 11.7271.3 16 Bi⁶⁺ 9.0 271.3 17 Tl⁺ 16.4 303.5 18 Tl³⁺ 10.3 303.5 19 Cd²⁺10.9 320.9 20 Pb²⁺ 13.3 327.502 21 Pb⁴⁺ 9.2 327.502 22 Zn²⁺ 8.8 419.5823 Te²⁻ 20.7 449.5 24 Te⁶⁺ 7.0 449.5 25 Sb³⁺ 9.0 630.74 26 Sb⁵⁺ 7.4630.74 27 Pu³⁺ 11.4 639.5 28 Pu⁴⁺ 10.0 639.5 29 Mg²⁺ 8.6 648.8 30 Al³⁺6.8 660.37 31 Ba²⁺ 14.9 725 32 Sr²⁺ 13.2 769 33 Ce³⁺ 11.5 799 34 Ce⁴⁺10.9 799 35 As³⁺ 7.2 817 36 As⁵⁺ 6.0 817 37 Eu²⁺ 13.1 822 38 Eu³⁺ 10.9822 39 Ca²⁺ 11.4 839 40 La³⁺ 11.7 921 41 Ge²⁺ 8.7 937.4 42 Ge⁴⁺ 6.7937.4 43 Ag⁺ 12.9 961.93 44 Ag²⁺ 10.8 961.93 45 Nd³⁺ 11.2 1020 46 Ac³⁺12.6 1050 47 Au⁺ 15.1 1064.43 48 Cu⁺ 9.1 1083.4 49 Cu²⁺ 8.7 1083.4 50U³⁺ 11.7 1132.3 51 U⁴⁺ 10.3 1132.3 52 U⁵⁺ 8.7 1132.3 53 Mn^(2+l) 8.11240 54 Mn^(2+h) 9.7 1240 55 Mn^(3+l) 7.2 1240 56 Mn^(3+h) 7.9 1240 57Mn⁴⁺ 6.7 1240 58 Be²⁺ 5.9 1280 59 Gd³⁺ 10.8 1310 60 Dy³⁺ 10.5 1410 61Si⁴⁺ 5.4 1410 62 Ni²⁺ 8.3 1450 63 Ni^(3+l) 7.0 1450 64 Ni^(3+h) 7.4 145065 Co^(2+l) 7.9 1490 66 Co^(2+h) 8.9 1490 67 Co^(3+l) 6.9 1490 68Co^(3+h) 7.5 1490 69 Y³⁺ 10.4 1520 70 Sc³⁺ 8.8 1540 71 Fe^(2+l) 7.5 154072 Fe^(2+h) 9.2 1540 73 Fe^(3+l) 6.9 1540 74 Fe^(3+h) 7.9 1540 75 Pd²⁺10.0 1550 76 Pd³⁺ 9.0 1550 77 Pd⁴⁺ 7.6 1550 78 Lu³⁺ 10.0 1660 79 Ti²⁺10.0 1660 80 Ti³⁺ 8.1 1660

For example, when the base material comprises a Ge₂Sb₂Te₅ composition,Al is a suitable element that can satisfy the two conditions concerningion radius and melting point simultaneously, while it is free oftoxicity or radioactivity. A GeTe—Sb₂Te₃-based composition can betreated in the same manner as Ge₂Sb₂Te₅. While the melting point of theGeTe—Sb₂Te₃-based based composition changes continuously in a range from593° C. to 725° C., Al was effective as well in filling lattice defects.Similarly, in any material compositions based on Ge and Te, Al waseffective in filling lattice defects. Needless to say, elements otherthan Al were confirmed to enter lattices. It was confirmed that Ag, Cr,Mn, Sn, Pb, Mo In and Se enter lattices.

Elements to fill lattice defects are not limited to one kind, but pluralkinds of elements can be filled simultaneously. In an experimentperformed by the inventors, the crystallization speed was improvedremarkably by, for example, filling Sn (or Pb) in lattices when thematerial is Ge—Sb—Te based material or Ge—Bi—Te based material. Therepeatability was improved by filling Cr in lattices. Therefore, thecrystallization speed and repeatability were improved at the same timeby filling Sn (or Pb) together with Cr. Similar effects were obtained byfilling Mn in place of Cr in the crystal lattices. Filling Ag washelpful in improving optical reflectance variation between a crystallinephase and an amorphous phase (improvement in recording signalamplitude). Therefore, improvement in the recording signal amplitude andthe crystallization speed was achieved simultaneously by adding Ag andSn (or Pb) together. Signal amplitude and repeatability were improvedsimultaneously by filling Ag and Cr (or Mn) at the same time. Theaddition of Sn (or Pb), Ag and Cr (or Mn) together served to improvecrystallization speed, signal amplitude and repeatabilitysimultaneously.

FIG. 2 indicates a preferred embodiment for a recording layer used foranother optical information recording medium according to the presentinvention. FIG. 2 expresses schematically a partial microscopicstructure of a recording layer 8 at a laser irradiation part in any ofFIGS. 4A-4I. In FIG. 2, (a) denotes a crystalline phase (complex phase)100 comprising a mixture of a compound component 110 and an amorphouscomponent 120, while (b) denotes a single-amorphous phase 200. Therecording material layer is composed of the four elements of Ge, Sb, Teand Sn. The crystal component 110 in the complex phase 100 has a NaCltype structure comprising the four elements of Ge—Sb—Te—Sn. The 4 asites of the NaCl type structure (sites corresponding to Cl) areoccupied by Te, while the 4 b sites (sites corresponding to Na) areoccupied randomly by Ge, Sb and Sn. At the 4 b sites there are latticedefects to accept no atoms, which tends to decrease entire density. As aresult, volume variation between the crystalline phase and amorphousphase is decreased, and inconvenience such as deformation or perforationcaused by the phase change. is prevented. In the grain boundary,components that cannot enter the lattices exist in an amorphous state.Here, Sb exists in an amorphous state. It is preferable that an amountof the amorphous component is twice or less than the crystal componentby number of molecules. It is preferable A/C≦2, or more preferably,A/C≦1, where C denotes a number of molecules of the crystal componentand A denotes a number of molecules of the amorphous component. When theratio of the amorphous component exceeds twice, the crystallizationspeed will be lowered remarkably. On the other hand, when the ratio isclose to 0, the crystallization speed is increased excessively. It ispreferable that A/C≧0.01. The element that is found as an amorphouscomponent in the crystalline phase is not limited to Sb but it can beGe. Ge is effective in raising crystallization temperature or improvingrepeatability. The great viscosity of the amorphous Ge is considered toprovide such effects. It has been confirmed that elements such as Mn andCr can be added for depositing Ge.

From a macroscopic viewpoint, all elements are arranged in asubstantially uniform state in the single-amorphous phase 200. It isimportant for the recording film to change reversibly between the twostates during recording or rewriting information. At this time, it ispreferable that a part of the elements for forming the amorphous phase120 and elements for forming the compound component 110 in the complexphase 100 is common, so that the distance of atomic diffusion isdecreased at the time of phase change so as to complete the changerapidly. It is effective also in preventing generation of greatpositional compositional segregation when rewriting is repeated manytimes.

A material layer composing the recording layer comprises a material forforming a crystalline phase in a complex phase, and the material isrepresented by a format of Ma-Mb-Mc-α, in which Ma comprises Ge and atleast one of Sn and Pb, Mb comprises at least one of Sb and Bi, and Mccomprises at least one of Te and Se. Any other elements can be added ifrequired. For example, Mn, Cr, Ag, Al, In or the like can be added. Fora material for forming an amorphous phase in the complex phase, Sb or Geis suitable for a Ge—Sb—Te based material, while Ge or Bi is suitablefor a Ge—Bi—Te based material. For a AgInSbTe based material, In can beused.

In general, protective layers 9 and 10 in FIGS. 4B-4I are made of adielectric material. Protective layers suggested as optical disk mediain conventional techniques can be used as well. The examples include amaterial layer of an oxide alone or a complex oxide of an elementselected from Al, Mg, Si, Nb, Ta, Ti, Zr, Y, and Ge; a material layer ofa nitride or a nitride-oxide of an element selected from Al, B, Nb, Si,Ge, Ta, Ti, and Zr; a sulfide such as ZnS and PbS; a selenide such asZnSe; a carbide such as SiC; a fluoride such as CaF₂ and LaF; and amixture thereof such as ZnS—SiO₂ and ZnSe—SiO₂.

A reflecting layer 11 is based on a metal such as Au, Al, Ag, Cu, Ni,Cr, Pd, Pt, Si, and Ge, or an alloy such as Au—Cr, Ni—Cr, Al—Cr, Al—Ta,Al—Ti, Ag—Pd, Ag—Pd—Cu, Si—W, and Si—Ta.

An overcoat layer 12 can be made of, for example, a photo-curable resin.An adhesive 13 can be made of, for example, a hot-melt adhesive or aphoto-curable resin such as an ultraviolet curable resin. A protectiveplate 14 can be made of the same material as the substrate. Thesubstrate is not transparent necessarily for a constitution to recordand reproduce by irradiating a laser beam from the side having arecording layer. The above-mentioned substrate can be replaced by, forexample, a plate of a light metal such as Al and Cu, or a plate of alloybased on the light metal, and a plate of ceramics such as Al₂O₃ andMgO₂. In this case, the respective layers are formed on the substrate ina reversed order.

Though it is not indispensable, a surface layer 15 can be provided onthe outermost in order to prevent damage caused by a contact with anoptical head. The surface layer can be made of a lubricant materialcomprising e.g., a diamond-like-carbon and a polymer material.

Interface layers 16 and 17 can be formed in an interface between therecording layer and at least one of the protective layers for severalpurposes, such as preventing atomic diffusion in spacing between therecording layer and the protective layer. Especially, nitrides,nitride-oxides and carbides are suitable for the interface layer. Theexamples include materials of Ge—N—(O), Al—N—(O), Si—C—N, Si—C or thelike, and materials further including Cr, Al or the like, such as Ge—C—Nand Si—Al—. Optical absorption Aa of a recording layer in an amorphousstate can be decreased relatively with respect to optical absorption Acof the recording layer in a crystalline state by applying an opticalabsorption layer 18 over an upper protective layer of the recordinglayer, or by applying a semitransparent reflecting layer 19 at the lightincident side of the recording layer.

The optical absorption layer can be made of alloy materials based on Siand Ge, or alloy materials based on Te. The reflecting layer can be madeof the same material, or it can be formed by laminating dielectric filmshaving different refractive indices, such as SiO₂/ZnS—SiO₂/SiO₂. Analternative medium can have both surfaces made by adhering a recordingmedium having these multilayer films 20 and 21 through adhesive layers13.

A multilayer film used for an optical information recording mediumaccording to the present invention can be formed by an ordinary methodfor forming a thin film. The method is selected, for example, frommagnetron sputtering, DC sputtering, electron beam deposition,resistance heating deposition, CVD, and ion plating. Especially,magnetron sputtering using an alloy target, and also DC sputtering areexcellent in obtaining uniform films that will be used as recordingfilms in the present invention. A target used for sputtering contains amain component of a material for forming the above-mentioned rock-saltstructure, to which an element for filling the lattice defects is added.Such a target can be prepared by solidifying powders composed ofrespective elements at a proper ratio, and the elements are, forexample, Ge, Te, Sb and Al; Ge, Sb, Sn, Cr and Te; Ge, Sb, Te, Sn andAg. Though the component ratio in the target substantially correspondsto compositions of the recording film, minor adjustment for everyapparatus is required since the components will be influenced by theapparatus. For example, Dad is equal substantially to Dim≦Ddf×1.5, whereDim denotes a concentration of an additive in a film of the crystallinephase, Ddf denotes a concentration of lattice defects, and Dad denotes aconcentration of an additive in a target. In general, an amorphoussingle phase is formed just after film formation, which will betransformed into a crystalline phase (initialization). It is possible toform a phase as a mixture of the crystalline phase and the amorphousphase by irradiating with a high density energy flux. In irradiation ofthe high density energy flux, it is desirable to penetrate the flux at ahigh temperature for a short period. Therefore, laser irradiation andflash irradiation are used suitably.

FIG. 10 is a schematic view to show a basic structure of an electricmemory device according to the present invention (a reversible changememory of a resistor). In FIG. 10, 23 is a substrate selected from aglass sheet, a ceramic sheet such as Al₂O₃, and sheets of various metalssuch as Si and Cu. The following explanation is about a case for usingan alumina substrate. In FIG. 10, an Au layer is sputtered to provide anelectrode 24 on a substrate. Subsequently, a layer 25 of an insulatorsuch as SiO₂ or SiN is formed thereon through a metal mask, and further,a recording layer 26 comprising a phase change material similar to theabove-mentioned recording layer for the optical information recordingmedium, and also an electrode (Au) 27 are laminated. Between theelectrodes 24 and 27, a pulse power source 30 is connected through aswitch 28. For crystallizing the recording film that is in highlyresistant under as-depo.-condition in order to change into a lowresistant state, the switch 28 closes (switch 29 open) so as to applyvoltage between the electrodes. The resistance value can be detectedwith a resistance meter 31 while opening the switch 28 and closing theswitch 29. For reversely transforming from the low resistant state to ahigh resistant state, voltage higher than the voltage at the time ofcrystallization is applied for the same or shorter period of time. Theresistance value can be detected with a resistance meter 31 whileopening the switch 28 and closing the switch 29. A large capacity memorycan be constituted by arranging a large number of the memory devices ina matrix.

The present invention will be described further by referring to specificexamples.

EXAMPLE 1

Example 1 is directed to a method for manufacturing an opticalinformation recording medium according to the present invention. Asubstrate used in this example was a disc-shape polycarbonate resinsubstrate that was 0.6 mm in thickness, 120 mm in diameter and 15 mm ininner diameter. A spiral groove was formed substantially on the wholesurface of the substrate. The track was a concave-convex groove having adepth of 70 nm. Both the groove portion and the land portion of thetrack had a width of 0.74 μm. A multilayer film would be formed on thesurface later. A laser beam for recording/reproducing an informationsignal can move to an arbitrary position on the disk by a servo signalprovided from the concave-convex shape. On the substrate, the followinglayers were formed in this order: a ZnS:20 mol % SiO₂ protective layer150 nm in thickness; a Ge₂Sb₂Te₅Al_(0.5) thin film 20 nm in thickness; aGeN interface layer 5 nm in thickness; a ZnS:20 mol % SiO₂ protectivelayer 40 nm in thickness; and an Al₉₇Cr₃ alloy reflecting plate 60nm inthickness. The protective layers were prepared by magnetron sputteringusing a ZnS—SiO₂ sintered target and Ar sputtering gas. The recordinglayer and the reflecting layer were prepared by DC sputtering in whichrespective alloy targets and Ar sputtering gas were used. The interfacelayer was formed by a reactive magnetron sputtering using a Ge targetand a sputtering gas as a mixture of Ar gas and N₂ gas. In any cases, N₂gas can be added to a sputtering gas. After completing the filmformation, an ultraviolet curable resin was spin-coated, and apolycarbonate plate the same as a substrate was adhered to serve as aprotective plate, and this was irradiated by a ultraviolet beam lampsubsequently for curing, before subjecting the disk to an initialcrystallization by irradiating a laser beam. The thus obtained opticalinformation recording medium can record and reproduce by means of laserirradiation. In an inspection with an X-ray diffraction, the part thatwas subjected to the initial crystallization was a NaCl typesingle-crystalline phase having Al in the crystal lattices, though aslight halo peak was observed. The same inspection was carried out forthe other additive elements, and similar results were observed for Mn,Ag, Cr, Sn, Bi, and Pb.

EXAMPLE 2

On a quartz substrate, eight kinds of thin film material were formed byDC sputtering. The materials were represented by Ge₂Sb₂Te₅Al_(x), inwhich Al:x=0.0, A2:x=0.2, A3:x=0.5, A4:x=1.0, A5:x=1.5, A6:x=2.0,A7:x=2.5, and A8:x=3.0. The base vacuum degree was 1.33×10⁻⁴ Pa, and Arwas introduced to make the vacuum degree to be 1.33×10⁻¹ Pa. Under thiscondition, 100 W power was applied between a cathode and an alloy targetof 100 mmΦ in diameter so as to form a thin film having a thickness of20 nm. These samples were monitored by using a He—Ne laser beam in thevarying strength of the transmitted light while being heated at aprogramming rate of 50° C./minute in order to measure a temperature atwhich transmittance was decreased remarkably as a result ofcrystallization. The results are shown in Table 3. TABLE 3 Relationshipbetween Al concentration in a Ge₂Sb₂Te₅ thin film and crystallizationtemperature · crystallization speed Sample A1 A2 A3 A4 A5 A6 A7 A8 Alcon.¹⁾ 0% 2.2% 5.3% 10% 14.3% 18.2% 21.7% 25% T_(x) 180° C. 183° C. 189°C. 200° C. 227° C. 255° C. 305° C. 350° C. T_(cry) ⊚ ⊚ ⊚ ⊚ ◯ Δ X X

The increase of the crystallization temperature becomes sharp when theAl concentration is at a level of the sample E. For this composition,Ddf (concentration of lattice defects) occupies 10% of the whole sites(20% of the 4 b sites). For the respective samples, ratios that Al atomsfill lattice defects to Ddf are as follows: A1:0, A2:0.2×Ddf,A3:0.5×Ddf, A4:1.0×Ddf, A5:1.5×Ddf, A6:2.0×Ddf, A7:2.5×Ddf, andA8:3.0×Ddf. For the samples A5-A8, there are more Al atoms than thelattice defects to be filled. Percentage of the Al atoms to the wholecompositions in the respective samples are as follows. A1: 0%, A2: 2.2%,A3: 5.3%, A4: 10%, A5: 14.3%, A6: 18.2%, A7: 21.7%, and A8: 25%.

Regarding the samples A3 and A4, a Rietveld method was performed toidentify the structures in detail by using an X-ray diffractometry so asto confirm that A1 entered the crystal sites in any of the samples. FIG.5 is a schematic view to show such a sample. The probability that thelattice defects are filled with the additives is determined randomly aswell. For the samples A5, A6, A7 and A8, excessive atoms that cannotenter the crystal lattices will exist among the crystal particles. Suchexcessive atoms are not always Al, but other elements such as Sb or Gemay deposit as a result of substitution with A1. Laser irradiationperiod for causing crystallization would be extended when the A1concentration is increased. In the Table, ⊚ indicates thatcrystallization occurred within 70 ns, ◯ indicates that crystallizationoccurred within 100 ns, Δ indicates that crystallization occurred within200 ns, and × indicates that crystallization required more than 200 ns.When an effective optical spot length is represented by 1/e², an idealvalue would be about 0.95 μm since an optical system used for thecurrent DVD-RAM has a wavelength of 660 nm, and NA of an objective lensis 0.6. It takes about 160 ns for the laser spot to traverse a diskrotating at a linear velocity of 6 m/s, which corresponds to a velocityfor DVD-RAM. Therefore, a disk with a ◯ mark can be applied to a currentDVD-RAM system. It can be applied to a system having a linear velocityof at least 9 m/s as well. A disk with ⊚ mark can cope with an evenhigher linear velocity of at least 12 m/s.

EXAMPLE 3

Eight optical disks from a1 to a8 were prepared by using thecompositions of Example 2 in the method of Example 1. These disk mediawere rotated at a linear velocity of 9 m/s, and light beams having awavelength of 660 nm emitted from a laser diode were focused on thedisks by using an optical system comprising an object lens having NA of0.6. At this time, as shown in FIGS. 6A-6C, overwriting recording wascarried out in a 8-16 modulation (bit length: 0.3 μm) by applying amulti-pulse waveform corresponding to waveforms of signals ranging froma 3T signal to a 11T signal. The peak power and bias power weredetermined as follows. First, a power to provide an amplitude of −3 dBto a saturation value of the amplitude was obtained and the power wasmultiplied by 1.3 to provide a peak power. Next, the peak power wasfixed while the bias power was determined to be variable for conducting3T recording. 11T recording was conducted with the same power formeasuring a damping ratio of the 3T signal, which was established as anerasing rate. Since the erasing rate was increased gradually,experienced a substantially flat region and turned into decrease, thebias power was determined to be a central value of the upper limit powerand a lower limit power with an erasing rate of more than 20 dB.

Table 4 shows recording power (peak power/bias power) at a time of landrecording for each disk, C/N, a maximum value for elimination rate, anda number of times that a jitter value is 13% or less when random signalsare overwrite-recorded repeatedly. TABLE 4 Relationship between Alconcentration in Ge₂Sb₂Te₅ thin film and disk performance Disk a1 a2 a3A4 a5 a6 a7 a8 Al con. 0% 2.2% 5.3% 10% 14.3% 18.2% 21.7% 25% Power10.5/4.5 mW 10.5/4.5 mW 10.5/4.5 mW 10.5/4.5 mW 10.1/4.6 mW 10.0/4.9 mW— — mW C/N 50 dB 51.5 dB 52 dB 52.5 dB 52.5 dB 52.5 dB 52.0 dB — Erasing25 dB   30 dB 34 dB   35 dB   29 dB   21 dB   10 dB — rate NT 3 × 10⁴ 1× 10⁵ >1 × 10⁵ >1 × 10⁵ 1 × 10⁵ 2 × 10⁴ — —¹⁾: Al concentration²⁾: Number of times

The results show that addition of Al improves erasing rate and increasesa number of repetitions. When the Al concentration was not higher than aconcentration (10%) of the lattice defects, erasing rates exceeded 30 dBand the repetition numbers exceeded 100,000 for any of the disks a2, a3,and a4. It was found that optimum values were obtained for C/N, erasingrate and repetition number when the Al concentration matches theconcentration Ddf of the lattice defects. High-speed crystallizationperformance was maintained up to the time that the Al concentrationbecame 1.5 times of the lattice defect concentration. For the disk a5,the repetition number was increased when compared to a disk including noadditives. When the additive concentration is increased excessively, thecrystallization velocity is lowered and thus, the erasing rate isdecreased and the jitter becomes large.

For the disks a7 and a8, the jitter was over 13% from the initial stage.It was observed for these disks having improved repeatability that masstransfer was restrained.

EXAMPLE 4

Various disks were manufactured by determining the composition of therecording film in Example 1 to be (GeTe)x(Sb₂Te₃)_(1−x), where the xvalue was varied in a range from 0 to 1. For every disk, D₁ and D₂ weremeasured. D₁ denotes a proper range of Al concentration, and D₂ denotesan optimum range among D₁. The concentration was determined first to be0.2% and 0.5%, and subsequently, it was increased by 0.5%, i.e., 1%,1.5%, 2%, 2.5%. The proper range was determined to be a concentrationrange to provide a repetition number larger than that of a diskincluding no additives, and the determination was based on the methodsdescribed in Examples 2 and 3. The optimum range was a concentrationrange in which the repetition number was doubled at least when comparedto a disk including no additives and a range that a high crystallizationvelocity was obtainable. Namely, it is a range to allow crystallizationby irradiating a laser beam for 150 ns at most. TABLE 5 Optimum Aladdition concentration for (GeTe)_(x)(Sb₂Te₃)_(1−x) Al concentrationwithin Al concentration within X value Ddf for NaCl structure properrange: D1 optimum range: D1 Notes 0 16.7% — — Sb₂Te₃ itself 0.1 16.1%0.2% ≦ D1 ≦ 24.0% 3.0% ≦ D2 ≦ 16.0% 0.2 15.4% 0.2% ≦ D1 ≦ 23.0% 3.0% ≦D2 ≦ 15.0% 0.33 14.3% 0.2% ≦ D1 ≦ 22.0% 3.0% ≦ D2 ≦ 14.0% GeSb₄Te₇ 0.512.5% 0.2% ≦ D1 ≦ 19.5% 2.0% ≦ D2 ≦ 12.5% GeSb₂Te₄ 0.67 10.0% 0.2% ≦ D1≦ 16.0% 1.5% ≦ D2 ≦ 11.0% Ge₂Sb₂Te₅ 0.8  7.1% 0.2% ≦ D1 ≦ 11.5% 0.5% ≦D2 ≦ 8.5% 0.9  4.2% 0.2% ≦ D1 ≦ 6.5% 0.2% ≦ D2 ≦ 4.5% 0.91  3.8% 0.2% ≦D1 ≦ 6.0% 0.2% ≦ D2 ≦ 4.0% 1   0% — — GeTe itself

Table 5 shows the test results. The table includes also calculationresults of the concentration Ddf of lattice defects. The lattice defectsare formed inevitably in a crystal structure under a hypotheticalcircumstance that these material thin films form metastable phases of arock-salt type by laser irradiation. As indicated in the table, theconcentration Ddf of the lattice defects increases when a(GeTe)_(x)(Sb₂Te₃)_(1−x) quasibinary system composition transfers fromthe GeTe side to the Sb₂Te₃ side. On the other hand, when the properrange of Al amount reaches a range higher than a range for the defectconcentration, the range up to about 1.5×Ddf is effective in improvingthe characteristics.

FIG. 7 is a graph to show the relationships. The solid line denotes Ddf,while ● denotes the upper limit of the proper range and Δ denotes theupper limit of the optimum range. The upper limit of the optimum rangesubstantially coincides with the Ddf value while the x value is smalland Ddf absolute value is big. However, the upper limit will be biggerthan Ddf by about 20% when the x value is increased and Ddf value isdecreased. The reason can be estimated as follows. Since a part of theAl additive is modified due to oxidization, nitriding or the like, apercentage for entering the crystal lattices is lowered, and thus, theamount of the additive should be increased.

EXAMPLE 5

Disks of Example 4 were subjected to 10000 times of overwrite-recordingof a single frequency signal having a mark length of 0.3 μm before ameasurement of the CN ratio. Subsequently, the disks were kept in athermostat at a temperature of 90° C. and humidity of 80% RH for 200hours and the CN ratio of the same track was measured. The results areshown in Table 6. In the table, ⊚ indicates that the initial CN ratiowas at least 50 dB and a decrease in the CN ratio was at most 1 dB evenafter a 200 hours of acceleration test. 0 indicates that the initial CNratio was at least 50 dB and a decrease in the CN ratio was at most 3 dBafter a 100 hours of acceleration test. Δ indicates that the initial CNratio was at least 50 dB while the CN ratio was decreased by at least 3dB in the acceleration test. × indicates that problems occurred duringthe initial overwriting of 10000 times, e.g., the CN ratio wasdecreased. TABLE 6 Result of acceleration test of disks based on(GeTe)_(x)(Sb₂Te₃)_((1−x)) containing Al X 0 0.1 0.2 0.33 0.5 0.67 0.80.9 0.91 1 Result Δ Δ ◯ ◯ ⊚ ⊚ ⊚ ⊚ X X

EXAMPLE 6

A similar test was carried out by changing the composition of therecording film of Example 4 to (GeTe)_(x)(Bi₂Te₃)_(1−x). Similar resultswere obtained for the effects caused by the Al addition and the properconcentration.

EXAMPLE 7

A similar test was carried out by changing the composition of therecording film of Example 4 to (GeTe)_(x)(M₂Te₃)_(1×x) (M: a mixturecomprising Sb and Bi at an arbitrary ratio). Similar results wereobtained for the effects caused by the Al addition and the properconcentration.

EXAMPLE 8

Disks having films with varied N concentration were prepared by varyingpartial pressures of Ar gas and N₂ gas, in which the recording layerswere formed by adding 7% Al to (GeTe)_(0.8)(Sb₂Te₃)_(0.2). Theconcentration of N in the films was identified by using SIMS. The thusobtained disks were subjected to recording of random signals having abit length of 0.26 μm under a condition that the recording power was 11mW (peak power)/5 mW (bias power) and the linear velocity was 9 m/s inorder to examine the overwriting characteristics. The evaluation resultsare shown in Table 7.

Table 7 indicates that addition of N improves recording sensitivity.When excessive N was added, the optical constant was reduced and C/N waslowered. The effects became apparent when 0.5% of N was added, and thepreferable amount of N was about 5%. TABLE 7 Relationship between Nconcentration in recording thin film and disk performance Disks A B C DE F G H N con. 0% 0.1% 0.5% 1% 3% 5% 10% 20% C/N 51.0 dB 51.0 dB 52.0 dB52.0 dB 52.5 dB 52.5 dB 49.5 dB 45.0 dB Power mW 11.5/5.0 mW 11.4/4.9 mW11.1/4.6 mW 10.8/4.4 mW 10.5/4.1 mW 10.0/4.0 mW 10.0/4.2 mW 10/4.4 mWN con.: N concentration

EXAMPLE 9

Various additives other than Al were added to Ge₂Sb₂Te₅ recording filmsfor the purpose of examining the recording performance of the films.Additives were selected from elements having ion radii similar to anionic radius of Al, i.e., V, S, Si, P, Se, Ge, Mn, Re, Al, Co, Te, Cr,Ni; elements having melting points similar to that of Al, i.e., Sb, Pu,Mg, Al, Ba; and elements of a separate group, i.e., Ag, Pb, and Sn. Eachadditive of about 5 atom % was added for examining the effects.

Disks were manufactured in accordance with Examples 1 and 3 in order toexamine the overwriting repeatability. Even if an element had an ionradius value similar to that of Al, the element often caused phaseseparation during repetition when the melting point is far from that ofAl. For an element having a melting point similar to that of Al,degradation occurred due to mass transfer as a result of repetition ifthe ion radius value was far apart from that of Al. When Pb or Sn wasadded, both the repeatability and crystallization speed were improved,while the crystallization temperature lowered to some degree. When Agwas added, the signal amplitude was improved, and the repetition numberwas increased slightly. In conclusion, a maximum repetition number wasobtained for a disk including an additive having an ion radius and amelting point similar to that of Al.

EXAMPLE 10

Various additives were added to Ge₃Al₂Te₆ recording films for thepurpose of examining the recording performance of the films. For theadditives, Sn, Pb and Ag were selected, since these elements will form arock-salt type crystal structure with Te (SnTe, PbTe, AgSbTe₂) in athermally equilibrium state. Concentrations of the respective elementswere 5% and 8.5%. Disks were manufactured in accordance with Examples 1and 3 for examining the laser crystal portions to find s rock-salt typecrystal of a single phase. In an examination on the overwritingrepeatability, no mass transfer occurred even after 10000 times ofrepetition.

FIG. 8A-8F and FIGS. 9A-9E show crystal structures for representativeexamples in Examples 10 and 11. In the drawings, only some of thestructures include lattice defects, which indicates that lattice defectsare formed depending on the compositions. Te or Se atoms occupy the 4 asites while the other atoms and lattice defects (vacancy) occupy the 4 bsites. The atoms occupy the respective sites at random and the rate isinfluenced by the composition.

EXAMPLE 11

A recording film was formed in which Sb of Example 4 was replaced by Al.The composition of the recording film was (GeTe)_(x)(Al₂Te₃)(_(1−x))(x=0.67, 0.8). The recording film was irradiated with a laser beam so asto obtain a metastable single phase. In an evaluation of the diskperformance, overwrite-recording at a linear velocity of 9 m/s wasachieved. Recording sensitivity was increased by about 10% in diskscomprising the composition together with 3 atom % of Sb or Bi.

EXAMPLE 12

In accordance with Example 1, various (100 kinds) optical disks weremanufactured in which the composition is represented by[(Ge+Sn)₄Sb₂Te₇](_(100−y))Cr_(y). In the composition, x indicates apercentage of Sn in the entire composition and y indicates atom %. Thevalues of x and y were varied in the following range:

-   -   x=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%    -   y=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%.        A substrate used in this example is a disc-shape polycarbonate        resin substrate that is 0.6 mm in thickness, 120 mm in diameter        and 15 mm in inner diameter. A spiral groove was formed on        substantially the whole surface of the substrate. The track was        a concave-convex groove having a depth of 70 nm. Both the groove        portion and the land portion of the track had a width of 0.615        μm. A multilayer film would be formed on the surface. A laser        beam for recording/reproducing an information signal can move to        an arbitrary position on the disk by a servo signal obtained        from the concave-convex shape. On the substrate, the following        layers were formed in this order: a ZnS:20 mol % SiO₂ protective        layer 100 nm in thickness; a GeN-based interface layer 5 nm in        thickness; a recording layer 9 nm in thickness having the        above-identified composition; a GeN interface layer 5nm in        thickness; a ZnS:20 mol % SiO₂ protective layer 40 nm in        thickness; a Ge-based or Si-based alloy layer 40 nm in        thickness; and an Ag-based metal reflecting layer 80 nm in        thickness. The disk characteristics were evaluated on three        criteria, i.e., signal volume, repetition number, and stability        of rewriting sensitivity (after an environmental test at 80° C.,        90% RH for 200 H ). In an evaluation carried out by taking a        disk of y=0 and z=0 as a standard, the crystallization speed was        increased with an increase of Sn concentration, while excessive        Sn decreased stability of an amorphous state. When Cr        concentration was increased, the crystallization speed and        signal amplitude were lowered and rewriting sensitivity was        lowered due to an environmental test, while the stability of the        amorphous state and repetition number were increased. It was        confirmed that equivalent or better performance was obtainable        for all the three criteria when the Sn concentration was in a        range from 3% to 15% and the Cr concentration was in a range        from 1% to 10%. It was effective especially in improving both        the repetition number and the stability of rewiring sensitivity        when the Sn concentration was in a range from 5% to 10% and the        Cr concentration was in a range from 1% to 5%.

EXAMPLE 13

In accordance with Example 12, 100 kinds of optical disks weremanufactured in which the composition is represented by[(Ge+Sn)₄Sb₂Te₇](_(100−z))Ag_(z). In the composition, x indicates apercentage of Sn in the entire composition and z indicates atom %. Thevalues of x and z were varied in the following range:

-   -   x=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%    -   z=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%.        The thickness of the respective layers and evaluation criteria        are identical to those of Example 12. It was confirmed that        crystallization speed was raised with an increase of Sn        concentration, but stability of an amorphous state deteriorated        when the concentration was increased excessively. It was        confirmed also that increase of Ag concentration increased        signal size, though excessive Ag lowered the repeatability.

It was confirmed that equivalent or better performance was obtainablefor all the three criteria in a comparison with a case where noadditives were included, when the Sn concentration was in a range from3% to 15% and the Ag concentration was in a range from 1% to 10%. It waseffective especially in improving both the signal amplitude and thestability of rewiring sensitivity when the Sn concentration was in arange from 5% to 10% and the Ag concentration was in a range from 1% to3%.

EXAMPLE 14

In accordance with Examples 12 and 13, 1000 kinds of optical disks weremanufactured in which the composition is represented by[(Ge+Sn)₄Sb₂Te₇](_(100y−z)z)Cr_(y)Ag_(z). In the composition, xindicates a percentage of Sn in the entire composition and y indicatesatom %. The values of x, y and z were varied in the following range:

-   -   x=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%    -   y=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%    -   z=0, 1, 2, 3, 4, 5, 8, 10, 15, 20%.        The thickness of the respective layers and evaluation criteria        are identical to those of Examples 12 and 13. It was confirmed        that equivalent or better performance was obtainable for all the        three criteria when the Sn concentration was in a range from 3%        to 15%, the Cr concentration was in a range from 1% to 5%, and        the Ag concentration was in a range from 1% to 10%. It was        effective especially in improving signal amplitude, stability of        rewiring sensitivity and repeatability when the Sn concentration        was in a range from 5% to 10%, the Cr concentration was in a        range from 1% to 3%, and the Ag concentration was in a range        from 1% to 3%.

EXAMPLE 15

Similar results were obtained in an evaluation in accordance withExamples 12, 13 and 14, where Cr was replaced by Mn.

EXAMPLE 16

The tests of Examples 12, 13, 14, and 15 were carried out afterreplacing the base material by a (GeTe)_(x)(Sb₂Te₃)(_(1−x)) quasibinarysystem material (0<x<1) and a GeTe—Bi₂Te₃ quasibinary system material(0<x<1), and similar effects were obtained. Particularly, when0.5≦x≦0.9, both the repeatability and amorphous stability wereobtainable. The Sn concentration was preferably ½ or less of the Geconcentration in the base material, since the amorphous phase stabilitydeteriorates when the Sn concentration exceeds the limitation.

EXAMPLE 17

On a 0.6 mm thick polycarbonate substrate, aGe₁₉Sn_(2.1)Sb_(26.3)Te_(52.6) (atom %) thin film having a thickness of1 μm was formed by sputtering. The whole surface of the film wasirradiated with a laser beam for crystallization, and subsequently, anx-ray diffraction pattern was observed and the structure was analyzed bya Rietveld method (a method to identify by measuring several modelsubstances and comparing the substances with a target substance) and aWPPF (whole-powder-peak-fitting) method. It was confirmed that the filmcomprised a NaCl type crystalline phase and amorphous phase, and thatthere were about 20% of lattice defects at the 4 b sites. Theabove-identified thin film composition can be represented by(Ge+Sn)₂Sb_(2.5)Te₅, in which about 0.5 mol of the 2.5 mol Sb cannotenter the lattices and the excessive Sb will be deposited as anamorphous component. At that time, the molar ratio (r) of thecomposition of the amorphous phase to that of the crystalline phase wasabout 0.5/1=0.5. In a test where the Sb concentration was varied on abasis of the composition, crystallization characteristics were keptexperimentally when ‘r’ was 2.0 or less. When ‘r’ was 1.0 or less, thecrystallization speed would be increased further.

EXAMPLE 18

Similar analysis was carried out by varying the composition of recordingfilms in Example 17. Table 8 shows the test results. The right column inthe table indicates speed of crystallization caused by laserirradiation. The mark ⊚ indicates that the time for crystallization is100 ns or less. ◯ indicates that the time is 200 ns or less, Δ denotesthat the time is 500 ns or less and × denotes the time exceeds 500 ns. Arecording film with a mark ◯ will be applied preferably to recentsystems, however, a recording film with a mark Δ also can be applied tothe systems. As indicated in the table, all of these compositionsinclude lattice defects inside thereof, and one phase forms a complexphase comprising a NaCl type crystalline phase and an amorphous phase.When a ratio ‘r’ of the amorphous phase to the crystalline phase in thecomplex phase is 1 or less, high speed crystallization is available.Crystallization will be difficult when the ratio ‘r’ exceeds 2. TABLE 8Compositions and structures of materials and crystallization performanceLattice Crystallization No. Total composition Structure of complex phasedefect r performance 1 Ge₃Sb_(2.5)Te₆ NaCl type crystalline phase 1mol + Sb 16% 0.5 ⊚ amorphous phase 0.5 mol 2 Ge₃Bi_(2.8)Te₆ NaCl typecrystalline phase 1 mol + Bi 16% 0.8 ⊚ amorphous phase 0.8 mol 3GeSb_(2.5)Bi₂Te₇ NaCl type crystalline phase 1 mol + Sb + Bi 28% 0.5 ⊚amorphous phase 0.5 mol 4 Ge₃SnBi_(2.7)Te₇ NaCl type crystalline phase 1mol + Sb 16% 0.7 ⊚ amorphous phase 0.7 mol 5 Ge₂Sb₂Cr_(0.3)Te₅ NaCl typecrystalline phase 1 mol + Sb 20% 0.3 ⊚ amorphous phase 0.3 mol 6GeSb₂In_(0.2)Te₄ NaCl type crystalline phase 1 mol + Sb 25% 0.2 ⊚amorphous phase 0.1 mol 7 GePb_(0.1)Bi₂Te₄ NaCl type crystalline phase 1mol + Bi 25% 0.1 ⊚ amorphous phase 0.1 mol 8 GeSb_(2.2)Se_(0.1)Te_(3.9)NaCl type crystalline phase 1 mol + Sb 20% 0.2 ⊚ amorphous phase 0.2 mol9 Ge_(3.5)Sn_(0.01)Sb₃Te₇ NaCl type crystalline phase 1 mol + Sb 16%0.01 ⊚ amorphous phase 0.01 mol 10 Ge_(3.5)Sn_(0.1)Sb_(3.5)Te₇ NaCl typecrystalline phase 1 mol + Sb 16% 0.3 ⊚ amorphous phase 0.3 mol 11Ge_(3.5)Sn_(0.5)Sb₃Te₇ NaCl type crystalline phase 1 mol + Sb 16% 1.0 ⊚amorphous phase 1.0 mol 12 Ge_(3.5)Sn_(0.5)Sb_(3.5)Te₇ NaCl typecrystalline phase 1 mol + Sb 16% 1.5 ◯ amorphous phase 1.5 mol 13Ge_(3.5)Sn_(0.5)Sb₄Te₇ NaCl type crystalline phase 1 mol + Sb 16% 2.0 Δamorphous phase 2.0 mol 14 Ge_(3.5)Sn_(0.5)Sb_(4.5)Te₇ NaCl typecrystalline phase 1 mol + Sb 16% 2.5 X amorphous phase 2.5 mol

EXAMPLE 19

A polycarbonate disk substrate having a diameter of 120 mm and thicknessof 0.6 mm was prepared, and a continuous groove 60 nm in depth and 0.6μm in width was formed on the surface. On this disk substrate, amultilayer film comprising the recording films of Nos. 9-18 in Example18 was formed in a predetermined order by sputtering, a protective platewas adhered by using an ultraviolet curing resin, and subsequently, therecording layers were crystallized by means of laser irradiation. Eachmultilayer film structure has six layer lamination on a substrate, andthe layers are ZnS—SiO_(2:) 20 mol % layer 90 nm in thickness, a Ge—Nlayer 5 nm in thickness, a recording layer 20 nm in thickness, a Ge—Nlayer 5 nm in thickness, a ZnS—SiO₂: 20 mol % layer 25 nm in thickness,and an Al alloy layer 100 nm in thickness.

A deck for evaluating the disk characteristics comprises an optical headequipped with a red semiconductor laser having a wavelength of 650 nmand an object lens having NA of 0.6. The rotation velocity of each diskwas varied to find the linear velocity range where recording and erasing(overwriting) were available. Modulation frequencies (f1 and f2) wereselected so that recording marks would be 0.6 μm and 2.2 μm under anylinear velocity conditions, and recording was carried out alternately inorder to find repeatability based on the C/N and the erasing rate. InExample 19, the recording portion was the groove. DC erasing was carriedout after the recording. The results are shown in Table 9. The linearvelocity demonstrated in Table 9 is the upper limit of linear velocityallowing the CIN that has been amorphous-recorded at f1 to exceed 48 dBand at the same time, the DC erasing rate (crystallization) of a f1signal to exceed 25 dB.

Table 9 shows that applicable range of linear velocity can be selectedcontinuously in an arbitrary manner in accordance with change of the rvalue. Under each maximum linear velocity condition, any disks providedexcellent repeatability of more than 10000 times. TABLE 9 Materialcomposition and limitation of applicable linear velocity Linear velocityNo. Composition R Repetition number limit 9 Ge_(3.5)Sn_(0.01)Sb₃Te₇0.01 >500,000 50.0 m/s 10 Ge_(3.5)Sn_(0.1)Sb_(3.5)Te₇ 0.3 >500,000 30.0m/s 11 Ge_(3.5)Sn_(0.5)Sb₃Te₇ 1.0 300,000 10.0 m/s 12Ge_(3.5)Sn_(0.5)Sb_(3.5)Te₇ 1.5 100,000  3.0 m/s 13Ge_(3.5)Sn_(0.5)Sb₄Te₇ 2.0 50,000  1.0 m/s 14Ge_(3.5)Sn_(0.5)Sb_(4.5)Te₇ 2.5 10,000  0.3 m/s

EXAMPLE 20

An apparatus as shown in FIG. 10 was assembled. In Example 20, a Sisubstrate having a nitrided surface was prepared. An electrode of Auhaving a thickness of 0.1 μm was provided on the substrate by sputteringand subsequently, a SiO₂ film having a thickness of 100 nm was formedthereon through a metal mask provided with a circular hole 0.5 mm indiameter. Next, a (Ge₃Sn₁Sb₂Te₇)₉₅Cr₅ film was formed thereon to have athickness of 0.5 μm, an Au electrode was sputtered to have a thicknessof 0.5 μm, and the respective electrodes were bonded to Au leads. Byapplying 500 mV voltage between these electrodes for a period of a pulsewidth of 100 ns, the device transformed from a high resistant state to alow resistant state. When this device was charged with current of 100 mAfor a period of a pulse width of 80 ns in the next step, the state ofthe device was reversed from the low resistant state to a high resistantstate.

Industrial Applicability

As mentioned above, the present invention provides an opticalinformation recording medium having a recording thin film. The recordingmedium having a recording thin film exhibits little variation of therecording and reproduction characteristics even after repetition ofrecording and reproduction, excellent weatherability. The presentinvention provides also a method of manufacturing the informationrecording medium. The present invention provides a recording mediumhaving a recording thin film that has strong resistance againstcomposition variation and easily controllable characteristics.

1. An information recording medium comprising a substrate and arecording material layer formed on the substrate, the recording materiallayer undergoing reversible phase change between electrically oroptically detectable states by electric energy or by electromagneticenergy, wherein the recording material layer comprises a materialselected from a material ‘A’ having a crystal structure comprising alattice defect in one phase of the reversible phase change; or amaterial ‘B’ in a complex phase composed of a crystal portion comprisinga lattice defect and an amorphous portion in one phase of the reversiblephase change, and the crystal portion and the amorphous portion comprisea common element; and at least a part of the lattice defect is filledwith an element other than an element constituting the crystalstructure.
 2. The information recording medium according to claim 1,wherein a molar ratio of the amorphous portion to the crystallineportion in the complex phase of the material ‘B’ is 2.0 at most.
 3. Theinformation recording medium according to claim 1, wherein thereversible phase change of the material ‘B’ occurs between the complexphase and a single phase.
 4. The information recording medium accordingto claim 1, wherein the crystal structure comprising the lattice defectis a NaCl type.
 5. The information recording medium according to claim1, wherein the crystal structure comprising the lattice defect comprisesTe or Se.
 6. The information recording medium according to claim 1,wherein the amorphous phase portion composing the complex phase of thematerial ‘B’ comprises at least one element selected from Sb, Bi, Ge andIn.
 7. The information recording medium according to claim 1, whereinthe crystal structure comprising the lattice defect comprises Ge, Sb andTe.
 8. The information recording medium according to claim 1, whereinthe crystal structure comprising the lattice defect comprises at leastone element selected from Ge, Sb, Bi and Te, and the amorphous componentin the complex phase comprises at least one element selected from Ge, Sband Bi.
 9. The information recording medium according to claim 7,wherein the crystal structure comprising the lattice defect furthercomprises at least one element selected from Sn, Cr, Mn, Ag, Al, Pb, Inand Se.
 10. The information recording medium according to claim 9,wherein the crystal structure comprising the lattice defect furthercomprises at least one combination of elements selected from Sn—Cr,Sn—Mn, Sn—Ag, Mn—Ag, Cr—Ag, and Sn—Cr—Ag.
 11. The information recordingmedium according to claim 1, wherein the element to fill at least a partof the lattice defect forms a stoichiometric rock-salt type crystal thatis stable with respect to Te.
 12. The information recording mediumaccording to claim 1, satisfying a relationship represented by 0.7Rnc<Rim≦1.05 Rnc, where Rim denotes an ionic radius of an elementfilling at least a part of the lattice defect, and Rnc denotes a minimumvalue of an ionic radius of an element constituting the crystalstructure.
 13. The information recording medium according to claim 1,satisfying a relationship represented by |Tim−Tnc|≦100° C. where Timdenotes a melting point of an element filling at least a part of thelattice defect, and Tnc denotes a melting point of a crystalconstituting the crystal structure.
 14. The information recording mediumaccording to claim 1, satisfying a relationship represented by 0.7Rnc<Rim≦1.05 Rnc and |Tim−Tnc|≦100° C., where Rim denotes an ionicradius of an element filling at least one part of the lattice defect,Tim denotes the melting point, Rnc denotes a minimum value of an ionicradius of an element constituting the crystal structure, and Tnc denotesthe melting point.
 15. The information recording medium according toclaim 1, satisfying a relationship represented by Dim≦Ddf×1.5, where Dimdenotes a concentration of an element added to fill the lattice defect,and Ddf denotes a concentration of the lattice defect in the crystalstructure.
 16. The information recording medium according to claim 15,wherein the Dim satisfies a relationship represented by 0.2≦Dim≦Ddf. 17.The information recording medium according to claim 11, wherein theelement to fill the lattice defect is at least one element selected fromAg, Sn and Pb.
 18. The information recording medium according to claim11, wherein the crystal structure comprising the lattice defect is atleast a group of elements selected from a GeTe—Sb₂Te₃ quasibinary systemcomposition, a GeTe—Bi₂Te₃ quasibinary system composition, and aGeTe—Al₂Te₃ quasibinary system composition.
 19. The informationrecording medium according to claim 18, wherein the element to fill thelattice defect is Al.
 20. The information recording medium according toclaim 18, wherein the crystal structure comprising the lattice defectcomprises (GeTe)(_(1−x))(M₂Te₃)_(x) where M denotes an element selectedfrom Sb, Bi, Al, and an arbitrary mixture of Sb, Bi, and Al; and xsatisfies 0.2≦x≦0.9.
 21. The information recording medium according toclaim 20, wherein x satisfies 0.5≦x≦0.9.
 22. The information recordingmedium according to claim 1, further comprising N in the recording film.23. The information recording medium according to claim 22, wherein aconcentration Dn of the N atom (atom %) is in a range of 0.5≦Dn≦5.
 24. Amethod for manufacturing an information recording medium having arecording material layer on a substrate, where reversible phase changebetween electrically or optically detectable states is caused byelectric energy or electromagnetic energy, wherein the recording layeris formed by using a recording material in which one phase of thereversible phase change comprises a lattice defect, and at least a partof the defect is filled with an additional element.
 25. The method formanufacturing an information recording medium according to claim 24,wherein after formation of the recording layer an element comprising thecrystal lattice is deposited outside the lattice by the additionalelement.
 26. The method for manufacturing an information recordingmedium according to claim 24, wherein the recording layer is formed bysputtering, and a sputtering target used in the sputtering comprises anelement constituting the crystal structure and the additional element.27. The method for manufacturing an information recording mediumaccording to claim 26, wherein a gas used in the sputtering comprises atleast one gas selected from N₂ gas and O₂ gas.